A conventional drum type washing machine is shown in FIG. 7. Below its configuration is described.
As shown in FIG. 7, drum 1 provided with multiple drum perforations 2 in its cylindrical surface is rotatably installed in water tub 3. One end of rotating shaft 4 is fixed at the center of rotation of drum 1, and its other end is connected to drum pulley 5. Motor 6 is connected to drum pulley 5 via belt 7, so that it can rotate drum 1. An opening of drum 1 is opened or closed by door 8. Water tub 3 is suspended from washing machine main body 9 via springs 10 and supported therein via vibration damper 11 in such a way that vibrations of water tub 3 are not transmitted to washing machine main body 9 during a water-extracting process. Further, water tub 3 is also provided with balancing weight 12 for absorbing the vibrations during the water-extracting process. Through water supply valve 13 wash water enters water tub 3, while through water drain pump 14 the wash water is goes out the tub. Heater 15 heats the wash water in water tub 3. Controller 16 controls operations of motor 6, water supply valve 13, water drain pump 14, heater 15 and so forth, to thereby control a series of operations including washing, rinsing and water-extracting processes in sequence (see, for example, Japanese Patent Laid-open Application No. H10-201988).
Here, the operation of a drum type washing machine with the above configuration will be described.
If a washing operation of the drum type washing machine is started after loading laundry into drum 1 through opening door 8, water supply valve 13 is opened, so that fresh wash water enters water tub 3. When the water level in water tub 3 reaches a predetermined level, water supply valve 13 is closed, and no more water enters the tub. Thereafter, drum 1 is rotated by motor 6 at a low rpm, during which the laundry loaded in drum 1 is lifted up and then plunged into the water over and over again, so as to loosen dirt. After a certain period of the wash cycle, water drain pump 14 is opened to drain soiled wash water from water tub 3, and an intermediate water-extracting operation is performed. Then, a rinse cycle is started, wherein the same operation as described in the washing process is performed. Thereafter, drum 1 is spun at a high rpm during a subsequent water-extracting cycle to wring out the water in the laundry using centrifugal force.
As for the conventional water-extracting process, although the drum is spun at a low rpm or subjected to an intermittent rotation in the initial stage of the water-extracting cycle to wring out wash water from the laundry, it has been impossible to completely eliminate residual water collecting at the bottom portion of the water tub due to various factors such as the water drain-hose layout or different laundry types which can affect the performance of the draining system.
Moreover, as for the draining conditions or factors affecting the drain behavior, they depend significantly on laundry types or operating conditions: the type and amount of detergent in the drum; soil types in the laundry. Because programming a water-extracting process based on such numerous factors is complicated, it is performed assuming the worst conditions for these factors. Consequently, the duration of the low-rpm or intermittent rotations of the drum is usually set to be excessively long, thereby undesirably extending the duration of the water-extracting process even if it may be unnecessary.
Further, if the water-extracting cycle is started while the clothes in the drum are entangled after the wash cycle, the overall weight distribution in the drum can be imbalanced during the water-extracting cycle. If the water-extracting process is performed in an off-balanced state, the drum will vibrate greatly as its rotational number hits the vibration resonance point of the water tub and others. As a result, excessive suds are generated as the wash water in the water tub contacts the drum, thereby causing an operational failure. Further, since the rotational number of the drum cannot be increased above the vibration resonance point, desired rinsing and water-extracting performance cannot be obtained.